Continuing failure to identify the source of the deadly German outbreak of E. coli poisoning points to a flawed investigation that could shake faith in the global public health system, a top U.S. food safety official said Monday.

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"All this wishy-washy back-and-forth, it's just incompetence," said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "Where's the epidemiology?"

German officials said Monday that initial tests showed that vegetable sprouts were not the cause of the food poisoning outbreak that has killed 22 and sickened at least 2,300, including more than 660 people with a life-threatening complication of infection that attacks the kidneys.

But Osterholm and other experts in the U.S. said that testing certain sprouts now — and finding they were negative for the outbreak strain of bacteria, E. coli 0104:H4 — could not rule them out as the source.

"All the culture tells you is that in the particular batch that you happen to have in your hand, it's not there," said Dr. Timothy Jones, the Tennessee state epidemiologist who serves on a federal food safety committee. "It doesn't mean the batch last week was clean or that the box next door was clean."

European food safety officials appear to rely far more on bacterial cultures than on tracing back what people involved in the outbreak actually ate — and where it came from. But a microbiological approach has repeatedly been shown to fall short of a detailed study of the epidemiology, or health patterns, that characterize foodborne illness outbreaks, Osterholm said.

Because vegetable sprouts and other produce are highly perishable, the timeframe for determining the source of the outbreak is dwindling rapidly, Jones said.

Waffling publicly over possible causes without solid explanations will likely shake public faith in all health officials' ability to keep the food supply safe, Osterholm said.

"The entire international public health credibility is on the line here," said Osterholm.

German officials did not immediately respond to requests for comments on the criticism.

The officials previously had targeted cucumbers, tomatoes and leafy greens. Studies released last week by the German Institute for Risk Assessment and the Robert Koch Institute showed some epidemiological evidence for those vegetables as the possible source.

Surveys of 46 infected people compared with 2,100 healthy control subjects showed that lettuce was eaten by 84 percent of those infected, but only 47 percent of controls.

Tomatoes were eaten by 80 percent of cases, but 63 percent of controls, while 75 percent of cases and 50 percent of controls had eaten cucumbers. It's not clear, however, whether the disparities are high enough to pin the outbreak on those vegetables.

And results for those who ate bean and vegetable sprouts were not reported, although those foods were targeted as sources over the weekend.

That kind of incomplete information can leave the public with too many questions, U.S. experts said.

"The more time that goes by and the bigger the outbreak is, the more we would expect answers," Jones said.